1. Field of the Invention
The present invention relates to a seam welding machine for welding flat plates, etc., by seam welding using electrical resistance, and in particular relates to a construction and control configuration for a circular electrode pressure cylinder and pressure roller cylinder.
2. Description of the Related Art
FIGS. 8(A) and 8(B) show a conventional seam welding machine such as that disclosed in Japanese Patent No. HEI 4-72638, for example. As shown in FIG. 8 (A), the conventional seam welding machine comprises:
a pair of upper and lower circular electrodes 104, 105 for applying pressure from above and below to an overlapping portion 103 where edges of two flat plate-shaped members being welded 101, 102 overlap and welding the members by imparting an electric current thereto while moving; PA1 a circular electrode pressure cylinder 106 for generating a force for pressing the upper circular electrode 104 against the overlapping portion 103 of the members being welded 101, 102; PA1 a pair of upper and lower pressure rollers 108, 109 for compressing the weld 107 welded by the upper and lower circular electrodes 104, 105 by applying additional pressure thereto from above and below; and PA1 a pressure roller cylinder 110 for generating a force for pressing the upper pressure roller 108 against the weld 107. PA1 a pressure-side direction-switching electromagnetic valve 125 for controlling the inflow and outflow of pressure fluid from a pressure source 124 to the pressure chamber 122; PA1 a back pressure-side direction-switching electromagnetic valve 126 for controlling the inflow and outflow of pressure fluid from the pressure source 124 to the back pressure chamber 123; and PA1 a pressure-side pressure-releasing valve 127 and a back pressure-side pressure-releasing valve 128 disposed in each of the circuits.
Electric current is supplied to the upper and lower circular electrodes 104, 105 from a welding transformer 141. These devices are mounted on a C-shaped frame 140.
As shown in FIG. 8(B), the members being welded 101, 102 are held by clamps 111, 112 in order to secure them against the separating forces generated in the weld 107 during welding. The lower circular electrode 105 and the lower pressure roller 109 are adjusted to the level of the clamps.
As shown in detail in FIG. 9, the circular electrode pressure cylinder 106 is provided with a freely reciprocating piston 113 having a piston rod 114 projecting downwards from the lower end of the circular electrode pressure cylinder 106, and the piston rod 114 is connected to a circular electrode pressure shaft 115 on the upper circular electrode 104. The circular electrode pressure shaft 115 is supported so as to freely reciprocate and is guided in the direction of application of pressure by means of circular electrode guide bearings 116.
Furthermore, the pressure roller cylinder 110 is provided with a freely reciprocating piston 117. The piston 117 comprises an upper piston rod 118 projecting upwards from the upper end of the pressure roller cylinder 110 and a lower piston rod 119 connected to the upper pressure roller 108 projecting downwards from the lower end of the pressure roller cylinder 110. A stopper 120 for limiting the descent of the upper pressure roller 108 is disposed on the upper end of the upper piston rod 118. The stopper 120 is designed to contact a gauge 121 whose position is adjustable from above, and the position of the gauge 121 is set so that the spacing between the upper pressure roller 108 at its limit of descent and the lower pressure roller 109 is a value corresponding to the final thickness of the finished plate at the weld 107.
Next, the control circuit of the above circular electrode pressure cylinder 106 will be explained with reference to FIG. 9.
The circular electrode pressure cylinder 106 is divided into a pressure chamber 122 on the opposite side of the piston 113 from the upper circular electrode 104 (the head side) and a back pressure chamber 123 on the same side as the upper circular electrode 104. The control circuit comprises:
Furthermore, the pressure-side direction-switching electromagnetic valve 125 comprises an inflow-side coil 125B for switching the direction of flow of pressure fluid so that fluid flows into the pressure chamber 122, and an outflow-side coil 125A for switching the direction of flow of pressure fluid so that fluid flows out of the pressure chamber 122. Furthermore, the back pressure-side direction-switching electromagnetic valve 126 comprises an inflow-side coil 126B for switching the direction of flow of pressure fluid so that fluid flows into the back pressure chamber 123, and an outflow-side coil 126A for switching the direction of flow of pressure fluid so that fluid flows out of the back pressure chamber 123.
When low pressure output is required such as in cases where the members 101, 102 being welded are thin plates, the inflow-side coil 125B of the pressure-side direction-switching electromagnetic valve 125 is energized allowing pressure fluid to flow into the pressure chamber 122 and the outflow-side coil 126A of the back pressure-side direction-switching electromagnetic valve 126 is energized allowing pressure fluid to flow out of the back pressure chamber 123, allowing the upper circular electrode 104 to descend, then pressure fluid is introduced to the back pressure chamber 123 of the circular electrode pressure cylinder 106 by re-energizing the inflow-side coil 126B of the back pressure-side direction-switching electromagnetic valve 126. The downward output (pressure output) of the circular electrode pressure cylinder 106 can be freely set if the pressure is low by means of the set value of the pressure-side pressure-releasing valve 127 and the back pressure-side pressure-releasing valve 128 in each of the circuits and by the pressure-receiving surface areas of the pressure chamber 122 and the back pressure chamber 123 in the circular electrode pressure cylinder 106. The above circuit is usually adopted because of problems such as poor performance of the pressure-releasing valves in the low pressure region when only the pressure of the pressure chamber 122 is being controlled which make it impossible to set the pressure at less than the weight of the moving portion of the circular electrode pressure cylinder 106.
Another plausible method of changing the pressure output of the circular electrode pressure cylinder 106 is a construction in which two cylinder portions having different pressure-receiving surface areas are disposed in series.
FIG. 10 shows a construction of a pressure cylinder 131 in which two cylinders 129, 130 are joined and a control system therefor such as that disclosed in Japanese Patent Laid-Open No. HEI 9-295159, for example.
In this conventional example, in addition to disposing the two cylinders 129, 130 on the longitudinal axis of a cylinder rod 132, two pistons 133,134 inserted into each of the cylinders 129,130 are formed integrally with the cylinder rod 132. The pressure output can be varied by changing the pressure-receiving surface areas of the two cylinders 129, 130. In other words, the overall pressure output of the pressure cylinder 131 is set by introducing compressed gas into a cylinder chamber 129a in one of the cylinders 129 by means of direction control valves 138, 139 as shown in FIG. 10(A) or by introducing compressed gas into a cylinder chamber 130a in the other cylinder 130 by means of the direction control valves 138, 139 as shown in FIG. 10(B).
However, when used in a seam welding machine, a conventional circular electrode pressure cylinder 106 such as that shown in FIG. 9 suffers from the problems described below.
It is necessary to set the pressure output of the upper circular electrode 104 at a value suited to the thickness and properties of the members being welded 101, 102. It is extremely important to maintain the applied pressure at the appropriate value because the applied pressure greatly affects the quality of the weld. However, in cases where the properties of the members being welded 101, 102, particularly the thickness thereof, vary over a wide range, when one tries to obtain low pressure for thin plates from the same circular electrode pressure cylinder 106 from which maximum output pressure is obtained for maximum plate thicknesses, the output pressure cannot be set accurately because resistance from sliding portions such as packing is constant regardless of the set value of the pressure output, making the influence of sliding resistance on pressure output greater in the low pressure regions corresponding to thin plates.
FIG. 11 is a diagram explaining the effects of sliding resistance on pressure output. In FIG. 11, the pressure output F.sub.0 acting at the point where the upper circular electrode 104 contacts the members being welded 101, 102 varies according to the values of the sliding resistance R.sub.0 of the packing of the piston 113, etc., and the sliding resistance R.sub.1 of the circular electrode guide bearings 116. During welding, the upper circular electrode 104 applies a pressure output F.sub.0 to the members being welded 101,102 as it moves horizontally (in the direction indicated by the arrow in the diagram). The piston 113 moves up and down due to deformation of the shape of outer circumference of the upper circular electrode 104 (deformation to a non-circular shape such as that indicated by the broken lines in the diagram, for example) or due to irregularities on the surfaces of the members being welded 101, 102. When the piston 113 is moving downwards, the sliding resistances R.sub.0, R.sub.1 act in a direction which subtracts from-the theoretical value of the pressure output F of the circular electrode pressure cylinder 106.
When the piston 113 is moving upwards, on the other hand, the sliding resistances R.sub.0, R.sub.1 act in a direction which adds to the theoretical value of the pressure output F of the circular electrode pressure cylinder 106. In other words, the variation in the value of the pressure output F due to these sliding resistances is 2.times.(R.sub.0 +R.sub.1.). Of these variations, the value of the sliding resistance R.sub.0 of the packing of the piston 113, etc., is generally constant depending on the circular electrode pressure cylinder 106. Consequently, in the conventional art such as that explained in FIG. 9, the proportion of variation in pressure output is extremely large when low pressure output is required, making it difficult to maintain weld quality for thin plates which require low pressure output.
Furthermore, in the conventional art such as that explained in FIG. 10, since the pressure cylinder 131 comprises two pistons 133, 134, sliding resistance in the pistons is increased further. Moreover, in this pressure cylinder 131, because two cylinders 129, 130 are formed around one cylinder rod 132, two shaft bushes and the two piston sliding portions mentioned above are disposed on the cylinder rod 132, making machining precision extremely difficult. If the concentricity at these four points is disturbed, sliding resistance increases and assembly is also made difficult.
The above is an explanation of problems with the pressure output of the circular electrode. Next, problems with the pressure output of the pressure cylinder will be explained.
As explained for FIG. 8, the thickness of the weld 107 is determined by setting the position of the gauge 121 to fix the position of the lower limit of the upper pressure roller 108, but due to the pressure output of the upper circular electrode 104 and the upper pressure roller 108, the frame 140 supporting them bends. One problem is that the value thereof varies depending on the value of the pressure output and causes variations in the finished thickness of the weld 107.
At the same time, another conventional method is known in which the weld is compressed by controlling pressure instead of controlling the position of the upper pressure roller 108 using the above stopper 120 and gauge 121.
However, in that case, one problem is that if pressure is applied to the members being welded 101, 102 before the upper pressure roller 108 mounts when the members being welded 101, 102 are thin, creases form at the ends of the members being welded 101, 102, and in worse cases the thin members being welded 101, 102 may be ruptured by the compressive force.
Thus, the pressure output has conventionally been applied after the upper pressure roller 108 mounts, but because the lag time is great between output of the pressure signal and actual application of pressure on the weld 107, a portion arises at the beginning of the weld 107 wherein the effect of the upper pressure roller 108 cannot be realized.
The length L thereof is given by L=t.times.V (here t is the lag time and V is the welding speed), and because the welding speed V must be changed depending on the members being welded 101, 102, it is necessary to reduce the lag time as much as possible in order to reduce the length L.